1. Field of Invention
This invention relates to using an optical photo sensor for measuring mechanical movement of a piston, rotor, joint, or other mechanical apparatus with reliable absolute position measurements obtained through repeated calibration of the optical photo sensor measured relative to the position of the piston, rotor, joint, or other mechanical apparatus.
2. Description of Prior Art
Measuring the absolute position of a piston relative to the cylinder is fundamental to control the operation of machinery. Correspondingly, industry has produced a variety of position sensing apparatuses using mechanical, magnetic, acoustic, and optical techniques for determining the instantaneous position of the movable piston or rotor.
One class of piston position sensing techniques is based on magnetic field sensors. U.S. Pat. No. 6,989,669 B2 forms a magnetically hard layer on the piston rod, and uses sensor to read the magnetic pattern recorded in the layer. However, the piston rod needs to be reproduced in order to form a magnetically hard layer on it, and the harsh environment may accidentally erase or alternate the magnetic pattern, which causes measurement unreliability. U.S. Pat. No. 6,690,160 B2 grooves the housing of the cylinder and mounts two magnetic members in the cylinder housing. Then, a magnetic field sensor generates signals which indicate the relative distance between the two magnetic members. Accordingly, the piston position is determined. This invention needs to groove the cylinder housing, which make the manufacturing and changing of component not easy. U.S. Pat. No. 5,201,838 uses a Hall effect transistor to sense the magnetic field which is generated by a permanent magnet. The sensed signal is used to determine the position of the piston. The accuracy of position measurement depends on the performance of the magnetic field sensor in use, and this class of position sensing technique is vulnerable in a strong magnetic environment.
Some inventions indirectly measure the piston position using various sensors. For example, U.S. Pat. No. 6,817,252 B2 uses a bi-directional flow sensor; U.S. Pat. No. 3,970,034 uses co-operating pairs of pressure sensors; U.S. Pat. No. 6,848,323 B2 measures the position based on a differential pressure flow sensor. U.S. Pat. No. 6,549,873 B1 senses the speed of ultrasonic wave and records time length. In U.S. Pat. No. 4,523,514, a potentiometric positioning sensing transducer is used, which is immunized to electrical noise. These sensors are readily available. However a complex detector means is required in order to obtain an accurate measurement. Moreover, correction required for accurate measurement requires additional sensors or apparatus, which increase the expenses. Even with complex additional sensors the accuracy of these sensing methods is very limited.
Resonant frequency techniques have been used in several inventions, such as U.S. Pat. Nos. 5,471,147, 5,438,274, 4,936,143, 5,617,034. The common feature of this class of position measurement methods is that a RF transmitting section and a receiving section are used to determine the resonant frequency of the cavity, which indicates the piston position. The RF signals in use include radio frequency signals, alternating pressure signals, and electromagnetic waves. Unfortunately oil is an efficient absorber of RF energy, as a result a significant portion of the transmitted RF energy is lost to heating the oil.
Piston position sensing techniques based on mechanical or electromechanical sensors were designed, for example, U.S. Pat. No. 5,438,261 uses a coil and an oscillator which produces a position signal as the reciprocating movement of the piston. U.S. Pat. Nos. 6,234,061 B1 and 6,694,861 B2 both use a non-contacting electromechanical transducer to provide an output signal proportional to the position or motion of the piston. However, these inventions need to mount the measurement apparatus in the cylinder, which makes manufacture and maintenance not easy. Moreover, extra power is needed to transmit and receive signals.
U.S. Pat. Nos. 5,977,778 and 6,722,260 B1 use the reflection of signals to measure the piston position in a cylinder. The signals in use include electromagnetic bursts and microwave pulses. The extension measurement directly depends on the transmitter and receiver. However, in order to obtain a more accurate measurement, more power is needed for signal transmitting and receiving. Moreover, the leaking of electromagnetic bursts or microwave pulses may be harmful to the surroundings, and the cylinder needs to be extensively modified to accommodate the sensing assembly, which causes relatively high complexity and cost, and relatively low reliability, durability, and accuracy.
Moreover, U.S. Pat. Nos. 4,814,553 and 7,268,341 provides an optical apparatus for determining the absolute position of a point on a surface or along a path including a tablet, scale, or overlay and a movable mouse-type cursor. The optical apparatus relies on markings added to the surface of the moving piston or rotor. These optical markings are costly to produce and are prone to rapid wear.